Electronically operated remote control systems, such as garage door openers, home security systems, home lighting systems, etc. are becoming increasingly common. Such electronic remote control systems typically employ a battery powered portable RF transmitter for transmitting a modulated and encoded RF signal to a receiver located at the remote control system. For example, a garage door opener system may include a receiver located within the homeowner's garage. The garage door receiver is tuned to the frequency of its associated portable RF transmitter and demodulates a predetermined code programmed into both the portable transmitter and receiver for operating the garage door. Conventional portable transmitters have consisted of a portable housing which typically is clipped to a vehicle's visor or otherwise loosely stored in the vehicle.
As an alternative to a portable transmitter, a trainable transceiver (e.g., a remote control transceiver) may be provided in vehicles for use with remote control devices such as garage door openers, gate controllers, alarm controls, home lighting systems, etc. Typically, a trainable transceiver may learn and store the modulation scheme (i.e., code format), transmission codes and the particular RF carrier frequencies of one or more OEM (original equipment manufacturer) remote transmitters for use with the remote control devices. A vehicle owner may train the transceiver to the vehicle owner's existing remote RF transmitter. Subsequently, the old remote RF transmitter can be discarded and stored. A trainable transceiver includes receiver circuitry to receive a control signal from a remote transmitter during a training process.
To enhance security of remote control devices, manufacturers have implemented cryptographic algorithms in their original transmitters and receivers that transmit and respond to randomly varying codes. To enable a vehicle-installed trainable transmitter to effectively operate in such systems, trainable transmitters have been developed that have the capability of recognizing when a received signal has been originated from a transmitter that generates a code that varies with each transmission in accordance with a cryptographic algorithm. When such a variable code is recognized, the trainable transmitter determines which cryptographic algorithm is used to generate and transmit the next code to which the receiver will respond.